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All galaxies began forming at about the same time approximately 13 billion years
ago. The origin of galaxies and how they changed over billions of years is
an active field of research in astronomy today. Models for galaxy formation
have been of two basic types: "top-down" and "bottom-up". The "top-down"
model on
the origin of the galaxies says that they formed from
huge gas clouds larger than the resulting galaxy. The clouds began collapsing
because their internal gravity was strong enough to overcome the pressure in
the cloud. If the gas cloud was slowly
rotating, then the collapsing gas cloud formed most of its stars
before the cloud could flatten into a disk. The result was an elliptical
galaxy. If the gas cloud was rotating faster, then the collapsing gas cloud formed
a disk before most of the stars were made. The result was a spiral
galaxy. The rate of star formation may be the determining factor in
what type of galaxy will form. But, perhaps the situation is reversed: the type
of
galaxy determines the rate of star formation. Which is the "cause" and which
is the "effect"?

A more recent variation of the "top-down" model says that there were
extremely large gas clouds that fragmented into smaller clouds. Each of the smaller
clouds then formed a galaxy. This explains why galaxies are grouped in
clusters and even clusters of galaxy clusters (superclusters). However, the
model predicts a very long time for the collapse of the super-large clouds and
fragmentation into individual galaxy clouds. There should still be galaxies
forming today. One galaxy called "I Zwicky 18" was recently discovered to be
a very young galaxy that began forming stars only 500 million years ago. However,
it is a dwarf galaxy whose formation might better be explained by the "bottom-up"
model called "hierarchical clustering".

I
Zwicky 18 is a nearby dwarf galaxy that started forming stars
only 500 million to one billion years ago.
It may be an example of what the galaxies were like over 12 billion years
ago. How this galaxy remained in an embryonic state for almost the entire
history of the universe is unknown. Is it an example of a "dark galaxy"---a
dark matter clump with cold primeval hydrogen and helium gas---in which
the gas
only now
got compressed
enough to form stars? One variation of the "botton-up" galaxy formation
model says that such "dark galaxies" are very numerous but very hard to see.

The "bottom-up" model builds galaxies from the merging of smaller clumps about the
size of a million solar masses (the sizes of the globular clusters). These
clumps would have been able to start collapsing when the universe was still
very young. Then galaxies would be drawn into clusters and clusters into
superclusters by their mutual gravity. This model predicts that there should
be many more small galaxies than large galaxies---that is observed to be true.
The dwarf irregular galaxies may be from cloud fragments that did not get
incorporated into larger galaxies.
Also, the galaxy clusters and superclusters should still be in the process of
forming---observations suggest this to be true, as well.

The radio
galaxy MRC 1138-262, also called the "Spiderweb Galaxy" is a large galaxy in the making. At 10.6 billion light years away, we see
it in the process of forming only 3 billion years after the Big Bang. Note
the small, thin "tadpole" and "chain" galaxies that are merging together
to create a giant galaxy.

Astronomers are now exploring formation models that combine
the "top-down" and "bottom-up" models. These models also incorporate
the dark matter that is now known to make up most of the mass of the universe.
Huge dark matter clumps the size of superclusters gather together under the
action of gravity into a network of filaments. Where the dark matter filaments
intersect, regular matter concentrates into galaxies and galaxy clusters.
The densest places would have had more rapid star
formation to make the elliptical galaxies while the lower density concentrations
would have made the spiral galaxies and dwarf galaxies. In such a model,
the visible galaxies ablaze in starlight are like the tip of an iceberg---the
visible matter is at the very densest part of much larger dark matter chunks.
Some dark matter clumps may have cold hydrogen and helium gas making "dark
galaxies" that have not become concentrated enough to start star formation---I
Zwicky 18 may be an example of one that did just recently.

A dark
matter map was published at the beginning of 2007 that
probed the dark matter distribution over a large expanse of sky and depth
(distance---see the "Distribution of Dark Matter" figure below). The map
is large enough and has high enough resolution to show the dark matter
becoming more concentrated with time. The map stretches halfway back to the
beginning of the universe. It also shows the visible matter clumping at the
densest areas of the dark matter filaments (see the "Distribution of
Visible and Dark Matter" figure below). The dark matter distribution
was measured by the weak gravitational lensing of the light from visible
galaxies by the dark matter (see the
relativity chapter).

Modern research into galaxy formation is also exploring the role that mergers
and collisions of galaxies plays in the structure of galaxies. The distances
between galaxies is large, but compared to the size of the galaxies, the
distances are not extremely large. Stars inside a galaxy do not collide because
the distances between them are hundreds of thousands to millions of times
larger than the sizes of the stars (recall the solar system as a quarter coin scale model from the previous chapter). The distances between galaxies are only
a few tens of times bigger than the galaxy sizes. Our Milky Way has several
small galaxies orbiting it that are one to two Milky Way diameters away. The
closest large galaxy, the Andromeda Galaxy, is only about 30 Milky Way
diameters away. Galaxies in rich clusters are closer to each other than that.

Collisions take place over very long timescales compared to the length of our
lifetime---several tens of millions of years. In order to study the collisions,
astronomers use powerful computers to simulate the gravitational interactions between
galaxies. The computer can run through a simulation in several hours to
a few days depending on the computer hardware and the number of interacting points.
The results are checked with observations of galaxies in different
stages of interaction. Note that this is the same process used to study the
evolution of stars. The physics of stellar interiors are input into the
computer model and the entire star's life cycle is simulated in a short time.
Then the results are checked with observations of stars in different stages
of their life.

In the past, computer simulations used several million points
to represent a galaxy to save on computer processing time.
A simulation of several million points could take many weeks to process.
However, galaxies are made of billions to trillions of stars,
so each point in the simulation actually represented large clusters of stars.
The simulations were said to be of "low resolution" because many individual
stars were smeared together to make one mass point in the simulation. The resolution
of a computer simulation does affect the result, but it is not known how much of
the result is influenced by the resolution of the simulation and how much the
ignorance of the physics plays a role. Computer hardware speeds and the programming
techniques have greatly improved, so astronomers are now getting to the point where
they can run simulations with several billion mass points in a few weeks time.

When two galaxies collide the stars will pass right on by each other without
colliding. The distances between stars is so large compared to the sizes of the
stars that star-star collisions are very rare when the galaxies collide. The
orbits of the stars can be radically changed, though. Gravity is a long-range
force and is the primary agent of the radical changes in a galaxy's structure
when another galaxy comes close to it. Computer simulations show that a
small galaxy passing close to a disk galaxy can trigger the formation of
spiral arms in the disk galaxy. Alas! The simulations show that the arms do
not last long. Part of the reason may be in the low resolution of the
simulations.

The stars may be flung out from the colliding galaxies to form long
arcs. Several examples of very distorted galaxies are seen with long
antenna-like arcs. In some collisions a small galaxy will collide head-on with
a large galaxy and punch a hole in the large galaxy. The stars are not destroyed.
The star orbits in the large galaxy are shifted to produce a ring
around a compact core.

Select the "Antennae Galaxies formation movie" link below to show a movie
of a computer simulation from Joshua
Barnes showing the formation of the Antennae Galaxies. It is a Quicktime
movie, so you will need a Quicktime viewer. The red particles are the dark matter
particles and the white and green are stars and gas, respectively. Other collision
movies are available on Barnes' Galaxy
Transformations web site and on Chris Mihos' Galaxy
Collisions and Mergers website.

Here are some photographs of examples of these collisions. The first is the
Antennae
Galaxies (NGC 4038 & NGC 4039) as viewed from the ground (left) and
from the Hubble Space Telescope (right). Note the large number of H II regions
produced from the collision. The second is the Cartwheel
Galaxy as seen by the Hubble Space Telescope. A large spiral was hit face-on
by one of the two galaxies to the right of the ring. The insets on the left
show details of the clumpy ring structure and the core of the Cartwheel. Selecting
the images will bring up an enlarged version in another window. See Mihos' galaxy
modelling website for simulations of the creation of the Cartwheel Galaxy.

The gas clouds in galaxies are much larger than the stars, so they will very
likely hit the clouds in another galaxy when the galaxies collide. When the
clouds hit each other, they compress and collapse to form a lot of stars in
a short time. Galaxies undergoing such a burst of star formation are called
starburst galaxies and they can be the among the most luminous of
galaxies.

Messier 82 (a starburst in the M81 group)

Though typical galaxy collisions take place over what to us seems a long
timescale, they are short compared to the lifetimes of galaxies. Some
collisions are gentler and longer-lasting. In such collisions the galaxies
can merge. Computer simulations show us that the big elliptical galaxies can
form from the collisions of galaxies, including spiral galaxies. Elliptical
galaxies formed in this way have faint shells of stars or dense clumps of stars
that are probably debris left from the merging process. Mergers of galaxies to
form ellipticals is probably why ellipticals are common in the central parts of
rich clusters. The spirals in the outer regions of the clusters have not
undergone any major interactions yet and so retain their original shape. Large
spirals can merge with small galaxies and retain a spiral structure.

Some satellite galaxies
of the Milky Way are in the process of merging with our galaxy. The dwarf elliptical
galaxy SagDEG in
the direction of the Milky Way's center is stretched and distorted from the
tidal effects of the Milky Way's strong gravity. The Canis
Major Dwarf galaxy about 25,000 light years from us is in a more advanced
stage of "digestion" by the Milky Way---just the nucleus of a former galaxy
is all that is left. A
narrow band of neutral hydrogen from other satellite galaxies, the Magellanic
Clouds, appears to be trailing behind those galaxies as they orbit the Milky
Way. The band of hydrogen gas, called the "Magellanic Stream", extends
almost 90 degrees across the sky away from the Magellanic Clouds and may be the
result
of
an encounter they experienced with the Milky Way about 200 million years ago.
At least eight other streams in the Milky Way from other dwarf galaxies have
been
found. The Andromeda Galaxy and the Milky Way will collide with each other 4 billion years from now and over the following two billion years after that initial encounter, they will merge to form an elliptical galaxy. The smaller spiral galaxy of the Local Group, the Triangulum Galaxy (M33) will also probably merge with us after that.

The giant ellipticals (called "cD galaxies") found close to the
centers of galaxies were formed from the collision and merging of galaxies.
When the giant elliptical gets large enough, it can gobble up nearby galaxies
whole. This is called galactic cannibalism. The cD galaxies will have
several bright concentrations in them instead of just one at the center. The
other bright points are the cores of other galaxies that have been gobbled up.

Messier 87 (a cD galaxy that has
grown large by swallowing smaller galaxies)

If collisions and mergers do happen, then more interactions should be seen when
looking at regions of space at very great distances. When you
look out to great distances, you see the universe as it was long ago because
the light from those places takes such a long time to reach us over the
billions of light years of intervening space. Edwin Hubble's discovery of the
expansion of the
universe means that the galaxies were once much closer together, so collisions
should have been more common. Pictures from the Hubble Space Telescope of
very distant galaxies show more distorted shapes, bent spiral arms, and
irregular fragments than in nearby galaxies (seen in a more recent stage of
their evolution).

The Hubble Ultra Deep Field---a narrow look back through time past many intervening
galaxies to the universe as it looked billions of years ago near the start of
the expansion. There
were
more
distorted
(interacting)
galaxies
back
then!
The
larger fuzzy patches in the picture are closer galaxies and the smallest
bright points are very distant galaxies. It is a 278-hour exposure (over 412
orbits)
of a single
piece
of sky in the Fornax constellation. In early 2010, astronomers announced that they were able to detect galaxies from the time of just 600 million to 800 million years after the birth of the universe (the Big Bang) using the new camera on the Hubble Space Telescope.

The formation of galaxies is one major field of current research in astronomy.
Astronomers are close to
solving the engineering problem of computer hardware speeds and simulation
techniques so that they can focus on the physical principles of galaxy formation.
One major roadblock in their progress is the lack of understanding of
the role that dark matter plays in the formation and interaction of galaxies.
Since the dark matter's composition is unknown and how far out it extends in the galaxies
and galaxy clusters is only beginning
to be mapped (and see also link), it is not known how
to best incorporate it into
the computer simulations. Faced with such ignorance of the nature of dark
matter, astronomers try inputting different models of the dark matter into
the simulations and see if the results match the observations. As mentioned in the previous section, models that use "cold dark matter" of "WIMPs" provide the best fit to the observed structures. The recent discovery of "dark
energy" is another major unknown in galaxy evolution
models, though its effect may be more important to the future of the universe
than to the origin and early history of the galaxies in which gravity and gas dynamics
played the significant role.

There will be many new fundamental discoveries made in the coming years, so
this section of the web site will surely undergo major revisions of the
content. Although the content of our knowledge will be changed and expanded,
the process of figuring out how things work will be the same. Theories and
models will be created from the past observations and the fundamental physical
laws and principles. Predictions will be made and then tested against new
observations. Nature will veto our ideas or say that we are on the right track.
Theories will be dropped, modified, or broadened. Having to reject a favorite
theory can be frustrating but the excitement of meeting the challenge of the
mystery and occasionally making a breakthrough in our understanding motivates
astronomers and other scientists to keep exploring.